1. Field of Invention
This invention relates to 2,5-bis(substituted aryl)thiazolo[5,4-d]thiazole compounds, and particularly to 2,5-bis(o-hydroxyaryl)thiazolo[5,4-d]thiazole color-formers, to their reactions with metal salts to form colored coordination compounds, and to imaging systems based thereon. The formation of colored coordination compounds can be employed to generate images and is important in the manufacture and use of pressure-sensitive transfer papers for preparing carbonless copies.
The invention also concerns the admixture of these color-formers with N-(monosubstituted)dithiooxamides and/or N,N'-(disubstituted)dithiooxamides to form images of various colors and preferably black images during the application of appropriate pressure to pressure-sensitive imaging constructions such as carbonless paper constructions.
2. Background of the Art
The use of coordination compounds to form imaging sheets has been important in the field of pressure sensitive transfer papers useful for preparing carbonless copies. The present invention provides color-forming compositions which, when complexed with transition metal ions, can provide compositions that exhibit light absorption characteristics such that they appear as intensely green-yellow colored complexes. This is accomplished in the present invention by the use of certain colorless 2,5-bis(substituted aryl)thiazolo[5,4-d]thiazole compounds, and particularly to certain 2,5-bis(o-hydroxyaryl)thiazolo[5,4-d]thiazole compounds which provide an intense green-yellow color when individually complexed with cations of certain transition metals as, for example, nickel.sup.2+.
An early preparation of a thiazolo[5,4-d]thiazole was reported by Ephraim (see Ephraim, J. Chem. Ber. 1891, 24, 1027) in which benzaldehyde was reacted with dithiooxamide to obtain a crystalline product in 25% yield. Later, Johnson and Ketcham identified the product as 2,5-diphenylthiazolo[5,4-d]thiazole (see Johnson, J. R. and Ketcham, R. J. Amer. Chem. Soc. 1960, 82, 2719). These workers also prepared a number of other derivatives using substituted benzaldehydes such as o-, m-, and p-hydroxy benzaldehyde, o- and p-methoxy benzaldehyde, furfuraldehyde and cinnamaldehyde. Additional substituted benzaldehydes were also used. These workers also prepared unsymmetrical 2,5-bis(aryl)thiazolo[5,4-d]thiazoles in which one of the aryl rings contained an o-hydroxy group and the other contained an o-ethoxy group.
The ultraviolet and fluorescent spectra of several thiazolo[5,4-d]thiazole derivatives including those from furfural and cinnamaldehyde have been studied (see Thomas, D. A. J. Hetercycl. Chem. 1970, 7, 457).
Sonnenfeld reacted dithiooxamide with a terephthaldehyde to give a reddish-brown self-extinguishing polymer with very good high temperature properties (see Sonnenfeld, R. J. U.S. Pat. No. 3,457,231).
Sawdey describes the use of 2,5-bis(substituted aryl)thiazolo[5,4-d]thiazole compounds in photographic elements as uv light absorbers (see Sawdey, G. W. U.S. Pat. No. 3,250,617.
Dear and coworkers taught the use of 2,5-bis(substituted aryl)thiazolo[5,4-d]thiazole compounds as both UV absorbers and fluorescent brighteners of both polymers and photographic elements (see Dear, K. M., et al. U.S. Pat. No. 3,630,738).
Dressler and coworkers dyed polypropylene yellow by treating polypropylene containing nickel with 2,5-bis(o-hydroxyphenyl)thiazolo[5,4-d]thiazole. The yellow color resulted from the formation of a nickel/2,5-bis(o-hydroxyphenyl)thiazolo[5,4-d]thiazole coordination compound (see Dressler, H. et al. U.S. Pat. No. 3,326,627).
In none of the above cited literature have 2,5-bis(substituted aryl)thiazolo[5,4-d]thiazole compounds been employed as color-forming ligands to form coordination compounds that provide the basis for an image forming process. That these color-formers can be encapsulated and can be used to form images in pressure sensitive carbonless imaging systems and particularly in combination with other ligands to form various colors in an imagewise fashion also appears new.
Carbonless impact marking papers for the transfer of images, (i.e., carbonless copy papers) are papers which are capable of producing an image upon application of pressure. Products employing this chemistry, generally comprise at least two substrates (for example two sheets of paper) and involve coating one reactant, known as a color-former, on one substrate, and the other reactant, known as a developer, on another, mating, substrate. One surface, or side, of each substrate is coated with one of the two primary reactants. The two substrates are often referred to as a donor sheet and a receptor sheet. Means for preventing the reaction of the two until intended, i.e., until activating pressure is applied, are also provided. This is typically accomplished by encapsulation of one of the reactants. Preferably, a fill solution of the color-forming compound(s) in a hydrophobic solvent are encapsulated or contained in microcapsules and is coated on the back side of one sheet of paper to form a donor sheet. This is then mated with a receptor sheet coated with a developer or reactant for the color-forming compound. The microcapsules serve the purpose of isolating the reactants from one another and preventing reaction. Once activating pressure is applied to the untreated surface of the donor sheet, as from a stylus or business-machine key, the two substrates come into contact under sufficient pressure so that the capsules are rupture in a pattern corresponding to the pattern of applied pressure, and the solution of encapsulated color-former is released and transferred from the donor sheet to the receptor sheet. On the receptor sheet, a reaction between the previously separated reactants occurs. Since the color-former and the developer form a deeply colored image when reacted, an image forms on the receptor sheet. In general, the resulting reaction will form a colored image corresponding to the path traveled by the stylus, or the pattern of pressure provided by the stylus or key. Herein the term, "activating pressure" includes, but is not limited to, pressure applied by hand with a stylus or pressure applied by a business machine key, for example a typewriter key; and the term "encapsulation" and "encapsulated compounds" refer to microcapsules enclosing a fill material therewithin.
A preferred construction comprises an encapsulated color-former dissolved in appropriate hydrophobic solvent(s) within microcapsules and coated onto a back side of the donor sheet with a suitable binder. The back side of the donor sheet is sometimes referred to herein as a "coated back" (CB) sheet. A developer, also optionally in a suitable binder, is coated onto the front side of the receptor sheet herein sometimes referred to as a "coated front" (CF) sheet. Herein, the term "suitable binder" refers to a material, such as starch or latex, that allows for dispersion of the reactants in a coating on a substrate, and is readily rupturable under hand-held stylus pressure, or typical business machine key pressure. As stated previously, in imaging, the two sheets are positioned such that the back side of the donor sheet faces the developer coating on the front side of the receptor sheet. In many applications the uncoated surface of the donor (CB) sheet comprises a form of some type and the activating pressure is generated by means of a pen or other writing instrument used in filling out the form. Thus, the image appearing on the receptor (CF) sheet is a copy of the image applied to the top sheet.
Constructions comprising a first substrate surface, on which is coated the encapsulated color-former; and, a second substrate surface, on which is coated a developer; are often prepared. The coated first substrate surface is positioned within the construction in contact with the coated second substrate surfaces. Such a construction is known as a "set" or a "form-set" construction.
Substrates, with one surface on which is coated the encapsulated color-former, and a second, opposite, surface on which is coated a developer can be placed between the CF and CB sheets, in a construction involving a plurality of substrates. Such sheets are generally referred to herein as "CFB" sheets (i.e., coated front and back sheets). Of course, each side including color-former thereon should be placed in juxtaposition with a sheet having developer thereon. CFB sheets are also typically used in form-sets. In some applications, multiple CFB sheets have been used in form-sets. These contain several intermediate sheets, each having a developer coating on one side and a coating with capsules of color-former on the opposite side.
An alternative to the use of CB, CF, and CFB sheet is the self-contained (SC), or autogenous, carbonless paper in which both the color-former and developer applied to the same side of the sheet and/or are incorporated into the fiber lattice of the paper sheet.
There are many stringent requirements for a color-former. In order to be useful in one embodiment of an imaging construction, it is necessary that the color-former be capable of being encapsulated. In addition, the color-former must be soluble and non-reactive with the fill solvent used for the encapsulation, insoluble in the aqueous solution used as the dispersing phase, non-reactive with other color-formers present in the encapsulation medium, and non-reactive with the materials used to form capsule walls.
It is also desirable that the color-former be capable of rapidly forming a stable colored image upon contact with a developer on a receptor sheet. That is, the color should form nearly instantaneously, so that the image is rapidly formed as the stylus pressure is applied to the backside of the donor sheet. This will help ensure formation of an accurate, almost instantly readable copy. The image should also be relatively stable so that it does not substantially fade with time.
One type of carbonless imaging chemistry takes advantage of the fact that dithiooxamide compounds are encapsulable and react readily with many transition metal salts to form coordination complexes. The chemistry and characteristics of certain dithiooxamide materials have been used successfully as color-formers in commercially available carbonless paper products. Generally, these dithiooxamide compounds comprise symmetrically disubstituted dithiooxamide compounds and include N,N'-dibenzyldithiooxamide and N,N'-di(2-octanoyloxyethyl)dithiooxamide.
Transition metal salts used as developers to form coordination complexes with dithiooxamides which have been employed in the preparation of carbonless image transfer products or constructions are generally those comprising cations having a +2 valance state. Compounds with nickel, zinc, palladium, platinum, copper and cobalt all form such complexes with dithiooxamides. Many of these coordination complexes are deeply colored.
Due to the stoichiometry of the system (i.e., the metal salt is usually in excess since relatively little ligand is released), it is generally believed that the image formed on the receptor sheet after stylus pressure is applied to break the capsules and release the ligand, results from the formation of a complex between one molecule of color-forming ligand and 1 or 2 atoms of a metal having a +2 valence (as for example Ni.sup.2+). The counterion of the positively charged transition metal is usually the conjugate base of a weak acid and may facilitate removal of the two protons from the color-forming ligands, necessary for complexation with the M.sup.2+ cation.
In commercial applications nickel salts have been preferred as the transition metal salts. One reason for this is that nickel salts form a deep color when complexed with dithiooxamide ligands. The nickel salts are also substantially colorless, and thus do not alone impart color to the receptor (CF) sheet. A third reason is that nickel salts are relatively low in cost, in comparison to other transition metal salts that can be easily and safely handled and that form highly colored coordination complexes with dithiooxamides.
In some applications it is also desirable that the color of the complex be a deep, strong color that is not only pleasing to the eye, but that will exhibit good contrast with the paper, for purposes of later reading and/or photocopying. This has been one drawback with conventional carbonless paper arrangements, which use nickel salts complexed with disubstituted dithiooxamide ligands. The image imparted by the resulting coordination compound, under such circumstances, is generally referred to as blue/purple (b/p) or magenta. The more "red" character the coordination complex exhibits, generally the less contrasting and pleasing is the appearance. A dark, i.e., preferably black, blue, or blue-black, arrangement would be preferred.
One attempt to prepare a neutral black image using metal coordination chemistry of this type was provided by Yarian (see Yarian, D. R. U.S. Pat. No. 4,334,015). He found that the combination of certain aromatic-substituted hydrazones with dithiooxamides followed by encapsulation of the mixture provides a method of achieving a dark image. These hydrazones react with the metal on the receiving sheet to form intense green-yellow images. The green-yellow coordination compound thus formed, combined with the blue-purple image formed by the dithiooxamide (such as N,N'-di(2-octanoyloxyethyl)dithiooxamide and/or N,N'-(dibenzyl)dithiooxamide), results in an image that appears almost black to the observer.
Although this is a successful approach, Yafian's use of hydrazones still suffers from several drawbacks. The solubility of the hydrazones is not as great in the solvents generally used in the encapsulation process as are dithiooxamides. In addition, the initial image color of the coordination compound formed with N,N'-(disubstituted)dithiooxamides is brown and only after some time does the blue-black to black final image color form. Although much better than the blue-purple coordination compound formed with N,N'-(disubstituted)dithiooxamides, this mixture of green-yellow and blue-purple is a dark blue-black rather than the preferred neutral black.
Yarian also noted that the color of capsules prepared from hydrazone compounds was pH dependent and their color may change from essentially colorless at low pH to yellow at pH greater than 9.5 to 10. Yarian further noted that this color change is rapid and reversible upon lowering of the pH. Papers can be divided into classes depending upon their methods of manufacture, treatment and sizing. Among these classifications are acidic and alkaline papers. Encapsulated hydrazones when coated onto "alkaline paper" can form yellow colors.
In conventional impact imaging constructions, the capsules can be inadvertently raptured in steps such as processing, printing, cutting, packaging, handling, storing, and copying. In these situations inadvertent marking or discoloration (i.e., backgrounding) of the sheets results from inadvertent capsule rupture and transfer of the encapsulated material to the mating sheet where color formation occurs. The mount of inadvertent backgrounding has been reduced in such products by the use of a color control coreactant distributed externally among the capsules. This coreactant is capable of reacting with the contents of the ruptured capsules before transfer of said contents to the receptor sheet and formation of an undesired image (see Ostlie, D. A., U.S. Pat. No. 3,481,759). Ostlie discovered that addition of a small amount of a metal salt such as a zinc salt to the capsule coating prevents the formation of colored background. The zinc metal ion reacts with the accidently released dithiooxamide compound to form colorless coordination compounds and thus deactivates inadvertantly released dithiooxamide materials.
The use of Yarian's invention in combination with that of Ostlie is not possible as zinc forms yellow coordination complexes with the hydrazones of Yarian's invention. Thus, yellow color backgrounding still occurs on the backside of the sheet due to inadvertently ruptured capsules. It would be desirable to have a yellow color-former that could be successfully deactivated by the same method as that described by Ostlie's discovery. Then, the same method of deactivation of the yellow, magenta, and cyan color-formers released by inadvertent capsule rapture would be possible.
Another approach to formation of a black image employs an encapsulated mixture of an acid sensitive green-forming leuco dye and a dithiooxamide color-former. The receptor sheet is formulated to contain phenolic type acids in addition to the transition metal salts. In this system, pressure imaging results in the release of both acid sensitive leuco dyes and dithiooxamide materials. The nickel salt in the receptor sheet reacts with the dithiooxamide to form a purple color and the phenolic acid in the receptor sheet reacts with the acid-sensitive leuco to form a green color. Together they generate a black image. This approach, while successful, has several disadvantages. Heavy coatings to the papers are required as two separate chemistries are involved. Another drawback of this approach is that the rates of reaction for the two chemistries are different and must be balanced by adjustment of the ratios of the two chemistries in the paper construction.
Recently, a blue or blue-black image was achieved by employing encapsulated N-(monosubstituted)dithiooxamides compatible with the transition metal chemistry described above (see copending U.S. patent application Ser. No. 07/483,776, now U.S. Pat. No. 5,124,308, incorporated herein by reference). Preparation of these N-(monosubstituted)dithiooxamides is described in Olson, D. B., et al. U.S. Pat. No. 5,041,654 which is incorporated herein by reference for the disclosure and synthesis of these N-(monosubstituted)dithiooxamides. These may be used either alone or in admixture with N,N'-(disubstituted)dithiooxamides and can result in a cyan, blue, or blue-black image. A neutral black image would be preferred.
The ligands generally useful in carbonless paper constructions should also be relatively nonvolatile, so that free ligand, which would result from any inadvertently ruptured capsule, does not readily transfer from the donor sheet to the receptor sheet and form undesired spots of imaged area. That is, so that without the specific assistance of stylus or key pressure, transfer is not readily obtained.
It is also preferred that the ligands should be colorless, since the ligands are often encapsulated and coated onto the backside of a sheet, such as a form, which has printing on one or both sides thereof. This allows for good legibility of printing on the back side of the carbonless copy-paper sheets. This aspect is particularly important if the donor sheet comprises a top sheet for a stack of carbonless papers. Such sheets are often white, so that they can be readily identified as originals, can be readily photocopied, and can be easily read. The presence of color in the coating on the back side of this sheet would detract from the white colored "original" appearance and could make photocopying of this sheet troublesome.
While the above-described preferred characteristics have long been desirable, they have not been satisfactorily achieved with conventional reactants and conventional constructions. What has been needed has been suitable materials and arrangements for achieving the desired features described.